In telecommunications and mobile radio technology, a requirement exists that the circuits necessary for data transmission are to be realized as integrated circuits in semiconductor bodies. In this case, the development of radiofrequency assemblies is striving for an ever higher integration density and easier portability to more finely patterned semiconductor processes. The desire for easy portability is often impeded by the technology used heretofore, which is essentially based on analog signal processing. For this reason, circuits are increasingly being designed and implemented for radiofrequency assemblies which comprise fully digital signal processing. These circuits additionally have the advantage that they can already be integrated into existing digital elements.
One group of devices that enables fully digital signal processing is the digital phase locked loop (all-digital PLL). The latter is often suitable for use in reception paths of mobile communication devices in which the generation of a pure carrier signal is required.
The inventor is aware that some digital phase locked loops generate interference signals. This is effected primarily if the desired output frequency of the digital phase locked loop is close to an integer multiple of a reference frequency used.
FIG. 6a shows a frequency-power diagram of a digital phase locked loop. It shows the phase noise of the output signal generated by the digital phase locked loop as a function of frequency spacing. It can clearly be discerned that additional interference signals are generated at a spacing of 60 kHz, 120 kHz and 180 kHz from the carrier signal. Thus, by way of example, the interference signal SP1, at a spacing of 60 kHz from the carrier signal, reaches a magnitude of −20 dBc, relative to the resolution bandwidth RBW of the measurement. The further interference signals SP2 and SP3, too, which represent the harmonic components of the interference signal SP1 at 120 kHz and 180 kHz, are possibly not suppressed sufficiently with respect to the carrier signal. These interference signals mean that the transmission quality is impaired and require complex filter measures to suppress them.